1. Field of the Invention
This invention relates to improvements in the manufacture of footwear and has been devised with the main object of producing a shoe with an upper having an improved shape-retaining performance by means of an improved and novel mode of heat setting the lasted shoe.
2. Description of the Prior Art
This improved and novel heat setting mode has been devised as a result of a consideration of the performance in use of the heel region of a conventional shoe upper. In particular it is well known that the heel region of a shoe upper has to change shape to allow the foot to enter. This can result in distortion of the heel region, and distortion can also happen if the wearer's foot is significantly broader or of different shape than the last on which the shoe was made. It is desirable both for comfort and for appearance that the heel part of the shoe upper should return to its original shape and should resist this distortion to some degree. For this purpose it is the practice to heat set the shoe upper and to insert stiffeners in the heel and other parts of the shoe upper.
Stiffeners may take various forms and be made of various materials as summarized in the four following paragraphs.
1. Thermoplastic Flat Stiffeners are widely used. The stiffener is inserted between the lining and outside in the upper assembly in a flat form and it then has to be heated before lasting and set after lasting. There are disadvantages due to having to apply heat before lasting, in the stiffener yielding and distorting in wear because it is insufficiently strong, and in the difficulty of getting the stiffener to conform tightly to the last shape.
2. Premoulded Fibre/Leather Board Stiffeners.
These may have a thermally activated adhesive coating applied and may contain a small amount of Thermoplastic material. This stiffener is stronger but it is difficult to get the exact mould shape for each size and fitting of last, and it is difficult to insert it into the upper assembly and carry out the lasting operations in such a way that the shoe is correctly lasted. It is expensive if good quality leather board is used, and it is ineffective if cheaper board mixes are used. Also its thickness makes the seat of the shoe bulky in appearance.
3. Solvent Activated Flat Stiffeners are frequently used. These produce a very strong final stiffening shape, but they are very difficult to use in the lasting process as the solvent tends to strike through and damage the upper leather. It is also difficult to get this stiffener to correspond precisely to the last shape.
4. Plastic Moulded Stiffeners. This gives a very strong permanent shape to the shoe, and it may be so strong that it is uncomfortable to some wearers whose feet do not correspond with the last shape. The main disadvantages are that it is not easy to bond this stiffener to the upper or to the insole, and it is very expensive to provide metal moulds for the number of different shapes required.
Apart from the foregoing it is found that whilst conventional heat setting relaxes the strains in a lasted upper it also softens a thermoplastic stiffener. If too much tension has been applied in lasting and if the stiffener softens before the strains are relaxed distortion will take place. In conventional heat setting there is the danger that the upper (particularly the top line) may come away from the last contour.